In a mobile communication system employing the IMT-2000 (International Mobile Telecommunication-2000) scheme, the following is defined for lossless handover implementation in a case where a mobile station UE (User Equipment) performs handover (SRNS relocation) between radio base stations NodeB connected to two respective radio network controllers (RNC) connected under control of the same switching center SGSN (Serving GPRS Support Node). Specifically, firstly, protocol status synchronization (PDCP sequence number synchronization) is first performed between a handover-target radio network controller and the mobile station UE by using a Layer-2 sub-layer (e.g., PDCP: Packet Data Convergence Protocol). Then, a handover-source radio network controller forwards not-forwarded PDCP-PDUs (Protocol Data Units) which have been not forwarded to the mobile station UE yet, to the handover-target radio network controller.
Here, after the handover-source radio network controller completes forwarding of the PDCP-PDUs accumulated therein, the handover-target radio network controller requests the switching center SGSN to open a downlink U-plane.
Specifically, as FIG. 1 shows, in the IMT-2000 mobile communication system, upon start of the SRNS relocation, a handover-source radio network controller (Source RNC) transmits a relocation start message (Relocation Request) to a switching center SGSN by using RANAP (Radio Access Network Application Part) in Step S1. The transmission of the relocation start message serves as a trigger to stop forwarding of user data to the handover-source radio network controller (Source RNC).
In Step S2, the mobile station UE performs processing for handover from the handover-source radio network controller to a handover-target radio network controller (Target RNC). Then in Step S3, the handover-source radio network controller forwards the not-forwarded PDCP-PDUs to the handover-target radio network controller.
After completion of the PDCP-PDU forwarding, in Step S4, the handover-target radio network controller transmits a relocation completion command (Relocation Request Acknowledge) to the switching center SGSN.
The switching center SGSN is configured to open the U-plane between the switching center SGSN and the handover-target radio network controller by being triggered by the transmission of the relocation completion command. Accordingly, there has been a problem of a long interruption time in the handover processing.
On the other hand, there is a mobile communication system employing the LTE (Long Term Evolution) scheme, the 3GPP standardization of which is underway. What is discussed as to the LTE mobile communication system is that, when the mobile station UE performs handover processing, a handover-source radio base station (Source eNB) forwards stored user data for the mobile station UE to a handover-target radio base station (Target eNB), as FIG. 2 shows.
Non-patent Document 1: 3GPP TS25.323